Hitherto, electrochromic devices (hereinafter referred to as “EC devices”) have been known as devices such as light adjusters having a light adjusting capability of enabling the transmissivity to be freely adjusted (hereinafter referred to as “light adjusting devices”). Such an EC device is, for example, made from a material that undergoes a spectral change upon an electrochemical redox reaction, such as tungsten oxide or prussian blue, and controls the amount of light transmitted by absorbing light. However, because such an EC device is current driven, if the EC device is made to have a large area, then a large voltage drop arises, the response speed drops markedly, and degradation of constituent materials due to electrochemical changes and so on occurs during prolonged passing of a current; such an EC device thus cannot be used as a light adjuster that is required to be durable.
In recent years, voltage driven light adjusting devices have thus come to be used as light adjusters instead of such current driven EC devices. For example, nematic curvilinear aligned phase (NCAP) liquid crystal light adjusters are known as such voltage driven light adjusting devices. Such a nematic liquid crystal light adjuster is made from a liquid crystal material, and has excellent durability, and moreover can easily be made to have a large area (see, for example, Published Japanese Translation of PCT Application (Kohyo) No. S58-501631).
In general, such a light adjuster is comprised of a liquid crystal layer having a light adjusting function in which a plurality of voids are filled with liquid crystals, and a pair of PET films that sandwich the liquid crystal layer therebetween; transparent electrically conductive films are provided on respective facing surfaces of the PET films so as to be bonded to the liquid crystal layer, and a voltage is applied to the liquid crystal layer via this pair of transparent electrically conductive films. The liquid crystal layer is made of a transparent polymer film having a large number of voids therein, and each void is filled with liquid crystals so as to form a liquid crystal capsule.
According to such a light adjuster, when a voltage is not being applied, the liquid crystal molecules line up along the curved wall surfaces of the liquid crystal capsules, i.e. are not arranged along the direction of travel of transmitted light transmitted through the liquid crystal capsules; the optical path of the transmitted light thus meanders around, and incident light is scattered at boundary layers between the liquid crystal capsules and the polymer film, and hence the liquid crystal layer becomes translucent. On the other hand, when a voltage is applied, the liquid crystal molecules align to the direction of the electric field produced, and at this time, due to the liquid crystal layer being made from a material such that the ordinary refractive index no of the liquid crystal molecules matches the refractive index np of the polymer film, it becomes such that boundary layers between the liquid crystal capsules and the polymer film do not exist optically, and hence transmitted light incident on the liquid crystal layer can be transmitted unhindered, whereby the liquid crystal layer becomes transparent.
Due to the above principle, the light adjuster has a visual field controlling capability of blocking the visual field through scattering of incident light when a voltage is not being applied, and securing the visual field through allowing incident light to be transmitted unhindered when a voltage is being applied.
Due to this visual field controlling capability, such a light adjuster is suitable for use as a partition either alone or in the form of a laminated glass sandwiched between a plurality of glass plates; in recent years, however, there has been an increase in cases in which such a light adjuster is used as a projection screen.
An example in which such a light adjuster is used as a projection screen is a display window having a rear projector provided thereinside. According to such a light adjuster, when the light adjuster becomes transparent due to application of a voltage to the liquid crystal layer, merchandise displayed inside the display window is made visible, and when the light adjuster becomes translucent (non-transparent) due to a voltage not being applied to the liquid crystal layer, an image advertising the merchandise or the like projected onto the window from the rear projector provided inside the display window is cast out.
Here, a small degree of scattering remaining when the light adjuster is transparent is referred to as “haze”. In the case of use as a display window as described above, it is required to reduce the haze when transparent. Furthermore, a display window has a large opening area, and hence as well as reducing the haze when transparent, it is also required for the dependence of the haze on the viewing angle to be low.
Moreover, so that such a light adjuster can function as a projection screen onto which is cast an image projected from a rear projector or the like, it is also required to reduce the wavelength dependence of the transmissivity in a non-transparent state.
It is known that the dependence of the haze on the viewing angle when the light adjuster is made transparent using a sinusoidal power source can be reduced by using liquid crystals having a low birefringence (hereinafter referred to as “Δn”) (see, for example, Japanese Laid-open Patent Publication (Kokai) No. H03-116019).
However, for such a light adjuster, even though the dependence of the haze on the viewing angle in the transparent state can be reduced by using liquid crystals having a low birefringence Δn, the wavelength dependence of the transmissivity in the non-transparent state cannot be reduced; for example, there is a problem that light of long wavelength is preferentially transmitted, and hence the projection screen has a reddish tinge.
It is an object of the present invention to provide a light adjuster and a laminated glass which are capable of reducing the wavelength dependence of the transmissivity when a voltage is not being applied.